Polytetramethylene ether glycols (PTMEG) are primarily used as chain segments in polyurethanes and polyesters. The most important uses are in the polyurethane art, i.e., the production of spandex fiber, elastomers, and thermoplastics. PTMEG with molecular weights of 1000 and 2000 are commercially produced and utilized on a large scale. PTMEG with molecular weights of 650 and 2900 are manufactured only on a small scale, and other grades are sometimes produced as specialty products. The polydispersity, or index of molecular weight distribution, of all grades of crude PTMEG products is usually about 2.0, a level which is generally not suitable for industrial use. Further treatments are often used to narrow the molecular weight distribution of PTMEG before it is utilized.
The most commonly used commercial treatment to narrow the PTMEG molecular weight distribution is solution extraction. A number of solution extraction procedures have been previously proposed for obtaining PTMEG or other polymerizates as fractions with smaller polydispersities. U.S. Pat. No. 5,053,553, issued to Dorai, teaches the extraction of PTMEG with methanol, water, and a non-polar cosolvent, where the cosolvent has a solubility parameter between 7.30 and 8.20. Japanese Patent No. 60-42,421 (1985), assigned to Mitsubishi, teaches the separation using methanol and water. The mixed solvent and PTMEG is used at a weight ratio of greater than 1.2. Japanese Patent No. 60-108,424 (1985), also describes a separation technique utilizing methanol and/or ethanol and water.
German Patent No. 3,607,946 A1 (1987) teaches the solvent separation of polytetrahydrofuran (PTHF) and tetrahydrofuran/alkylene oxide copolymers using a 1-4 C alkanol, preferably methanol or ethanol, a 4-12 C hydrocarbon, preferably cyclohexane or methylcyclohexane, and water. German Patent No. 3,728,613 A1 (1989) discloses a similar method, but a 4-12 C aliphatic hydrocarbon, especially a 4-7 C linear aliphatic hydrocarbon, is substituted for the cyclic hydrocarbons of DE 3,607,946.
U.S. Pat. No. 4,500,705 (1985), issued to Coplein, teaches the reduction of the oligomeric cyclic ether content of tetrahydrofuran or related polymerizate by contacting the polymerizate with an aliphatic or cycloaliphatic hydrocarbon of 6-8 carbon atoms, and separating the two resulting phases. U.S. Pat. No. 4,500,705 (1985), also issued to Coplein, teaches the reduction of the oligomeric cyclic ether content of a tetrahydroran or related polymerizate by contacting the polymerizate with a gas in the supercritical state.
However, the above described solution extraction technology can only fractionate PTMEG or the related polymerizate into two, or at most, three fractions. The lowest obtainable polydispersity of these fractions is about 1.3. As a result, there are limits on the industrial use of PTMEG in polymer formation. In theory, more fractions and narrower molecular weight distributions could be achieved through application of this solution extraction technology. However, it is anticipated that the solvent recovery from each fraction would be difficult in practice, requiring large energy input. Therefore, there is a need for the development of an improved and industrially feasible method for producing fractions having polydispersities of less than about 1.3, preferably about 1.1.
A polymer consists of many similar macromolecules with different molecular weights. In any one solvent, these macromolecules have different solubilities. Fractions of the polymer with narrow molecular weight distribution can be obtained by extracting the polymer individually with multiple solvents chosen for their selective solvent power. Alternately, molecular weight fractions can be isolated using one solvent by differing the extraction conditions. It is this principle has been applied in developing the present invention. Liquid propane, which possesses different solvent powers at different operating conditions, is utilized for fractionating PTMEG.